No. 3: K-Computer (Japan)

K Computer, which held the title of world's fastest supercomputer from June 2011 until June 2012, is now the third-ranked system.

The Japanese supercomputer is capable of 10.51 petaflops. Hence the supercomputer's name: The letter "K" is short for the Japanese word "kei," which means 10 quadrillion.

Unlike the two computers ahead of it on the Top 500 list, the K Computer only has CPUs -- no GPU accelerators. That makes it a truly massive supercomputer: it comprises 864 computer racks, four times the number of racks in the Titan system.

K Computer is used at the Riken Institute for Physical and Chemical Research in Kobe, Japan. The Riken Institute shares its computing power for researchers' projects around the world.

Japan has the third-most supercomputers on the list, behind the United States and China, and ahead of the United Kingdom, France and Germany.